The invention is in the field of electronic reproduction technology and is directed to an engraving element of an electronic engraving machine for engraving printing cylinders for rotogravure.
In an electronic engraving machine, an engraving element with an engraving stylus as a cutting tool moves in the axial direction along a rotating printing cylinder. Controlled by an engraving control signal, the engraving stylus cuts cups arranged in an engraving raster into the generated surface of the printing cylinder engraving line by engraving line. The engraving control signal is formed by superimposition of a periodic raster signal with image signal values that represent the tone values to be reproduced between xe2x80x9cblackxe2x80x9d and xe2x80x9cwhitexe2x80x9d. For generating the engraving raster, the raster signal effects a vibrating lifting motion of the engraving stylus in the direction of the printing cylinder, whereby the image signal values corresponding to the tone values to be reproduced determine the engraving depths of the cups.
DE-A-23 36 089 discloses an engraving element with an electromagnetic drive element for the engraving stylus. The electromagnetic drive element is composed of a stationary electromagnet charged with the engraving control signal in whose air gap the armature of a rotatory system moves. The rotatory system is composed of a shaft, the armature, a bearing for the shaft and of a damping device. One shaft end merges into a resilient torsion bar chucked spatially fixed, whereas the other shaft end carries a lever to which the engraving stylus is attached.
In practice, the engraving stylus is often a prismatically ground diamond that has its shank secured to the lever of the engraving element. The engraving stylus is essentially composed of a front face with respect to the engraving direction and of an obliquely proceeding, back flank facing away from the face. The intersection lines between the face and the flank form the cutting tip of the engraving stylus. The angle between the obliquely proceeding flank and the tangential plane in the point of contact between cutting tip and generated surface of the printing cylinder is referred to as relief angle.
The relief angle of the engraving stylus limits the steepness with which the engraving stylus can penetrate into the printing cylinder without the flank placing itself against the walls of the cup. For engraving different engraving rasters, cups that are flattened in the engraving line direction and have a steep wall and elongated cups with a shallow wall are engraved. When engraving flattened cups, it can disadvantageously occur that the flank of the engraving stylus touches down onto the walls of the cups and the cutting process is interrupted. The result is that the engraving depths required for tonally correct engraving are not achieved and cups having an undesired, asymmetrical shape are engraved.
In order to also be able to engrave flattened cups with steep walls without falsification of the tone value, the relief angle would have to be correspondingly enlarged. However, it has been shown in practice that the risk of breakage of the cutting tip of the diamond increases given an enlarged relief angle. Frequent replacement of the engraving stylus as well as time-consuming and costly re-engravings would be the result.
It is an object of the invention to improve an engraving element of an electronic engraving machine for engraving printing forms such that, in particular, a tonally correct engraving is achieved even given flattened cups.
According to the present method and apparatus of the invention, an engraving element is provided for an electronic engraving machine for engraving printing cylinders. A rotary system oscillates by small angles. A drive system for the rotary system is provided. An engraving stylus is attached to the rotary system for engraving cups in the printing cylinder, the engraving stylus respectively implementing a stroke motion directed onto the printing cylinder for engraving a cup. For enlarging a relief angle of the engraving stylus, the engraving stylus is arranged tilted in an engraving direction of the cups with respect to a reference straight line that proceeds perpendicular to a generated surface of the printing cylinder.
The invention is explained in greater detail below on the basis of FIGS. 1 and 2.